The present disclosure relates to matrix acidizing of subterranean formations, and, more specifically, to treatment fluids that can eliminate or reduce the production of insoluble fluorosilicates and aluminosilicates that may occur in conjunction with an acidizing operation.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, sand control treatments, and the like. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof. Illustrative treatment operations can include, for example, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal, consolidation operations, and the like.
In acidizing operations, a subterranean formation containing an acid-soluble material can be treated with an acid to dissolve at least a portion of the material. Formation components of the formation matrix may comprise the acid-soluble material in some cases. In other cases, the acid-soluble material may have been deliberately introduced into the subterranean formation in conjunction with a stimulation operation (e.g., proppant particulates). Illustrative examples of formation components that may be dissolved by an acid include, for example, carbonates, silicates, and aluminosilicates. Dissolution of these formation components can desirably open voids and conductive flow pathways in the formation that can improve the formation's rate of hydrocarbon production, for example. In a similar motif, acidization may be used to remove like types of precipitation damage that can be present in the formation.
Carbonate formations often contain minerals that comprise a carbonate anion (e.g., calcite). When acidizing a carbonate formation, the acidity of the treatment fluid alone can be sufficient to solubilize the formation components. Both mineral acids (e.g., hydrochloric acid) and organic acids (e.g., acetic and formic acids) can be used to treat a carbonate formation, often with similar degrees of success.
Siliceous formations can include minerals such as, for example, zeolites, clays, and feldspars. Most sandstone formations, for example, contain about 40% to about 98% sand quartz particles (i.e., silica), bonded together by various amounts of cementing material including carbonates (e.g., calcite), aluminosilicates, and other silicates. As used herein, the term “siliceous” refers to a substance having the characteristics of silica, including silicates and/or aluminosilicates.
Acidizing a siliceous formation (e.g., a sandstone formation or a clay-containing formation) or a formation containing a siliceous material is thought to be considerably different than acidizing a carbonate formation. Specifically, the treatment of a siliceous formation with the treatment fluids commonly used for acidizing a carbonate formation may have little to no effect, because mineral acids and organic acids do not effectively react with siliceous materials. In contrast to mineral acids and organic acids, hydrofluoric acid can react very readily with siliceous materials to produce soluble substances. Oftentimes, a mineral acid or an organic acid can be used in conjunction with a hydrofluoric acid-containing treatment fluid to maintain the treatment fluid in a low pH state as the hydrofluoric acid becomes spent. In some instances, the low pH of the treatment fluid may promote initial silicon dissolution and aid in maintaining the silicon in a dissolved state. At higher subterranean formation temperatures (e.g., above about 200° F.), it may be undesirable to lower the pH much below about 1 due to mineral instability that can occur. Additionally, regardless of the formation temperature, corrosion can be an inevitable problem that occurs when very low pH treatment fluids are used.
Although low pH treatment fluids may be desirable to aid in silicon dissolution, precipitation of insoluble fluorosilicates and aluminosilicates can still become problematic in the presence of certain metal ions. Specifically, under low pH conditions (e.g., below a pH of about 3), dissolved silicon can react with Group 1 metal ions (e.g., Na+ and K+) to produce insoluble fluorosilicates and aluminosilicates. The terms “Group 1 metal ions” and “alkali metal ions” will be used synonymously herein. Other metal ions, including Group 2 metal ions (e.g., Ca2+ and Mg2+), may also be problematic in this regard. The precipitation of insoluble fluorosilicates and aluminosilicates can block pore throats and undo the desirable permeability increase initially achieved by the acidizing operation. That is, the formation of insoluble fluorosilicates and aluminosilicates can damage the subterranean formation. In many instances, the damage produced by insoluble fluorosilicates and aluminosilicates can be more problematic than if the acidizing operation had not been conducted in the first place. In contrast to many metal ions, ammonium ions (NH4+) are not believed to promote the formation of insoluble fluorosilicates and aluminosilicates. Accordingly, treatment fluids comprising an ammonium salt are frequently used in conjunction with acidizing a siliceous formation, as discussed further below.
Problematic alkali metal ions or other metal ions can come from any source including, for example, the treatment fluid, a component of the treatment fluid, or the subterranean formation itself. For example, the carrier fluid of a treatment fluid may contain some sodium or potassium ions unless costly measures (e.g., deionization) are taken to limit their presence. Alkali metal ions, in particular, are widely distributed in the environment and can be especially difficult to avoid completely when conducting a subterranean treatment operation. As discussed further below, a variety of strategies have been developed to address the most common sources of problematic metal ions encountered when conducting subterranean treatment operations.
One strategy that has been used with some success to avoid the damaging effects of metal ions includes introducing a sequence of pre-flush treatment fluids into the subterranean formation prior to performing an acidizing operation with a hydrofluoric acid-containing treatment fluid. For example, a pre-flush treatment fluid comprising a mineral acid or an organic acid can be used to dissolve acid-soluble formation components and remove at least a portion of the problematic metal ions from the formation. Thereafter, another pre-flush treatment fluid comprising an ammonium salt can be introduced into the subterranean formation to displace the remaining formation metal ions and leave the formation enriched in ammonium ions. Although this approach can be used successfully, it can considerably add to the time and expense needed to perform an acidizing operation.
Another strategy that can be used to mitigate the effects of metal ions in acidizing operations is to introduce a chelating agent into the subterranean formation. Although this strategy can be successful for Group 2 metal ions and transition metal ions, for example, chelation is believed to be somewhat less effective for alkali metal ions. In addition, many chelating agents are utilized in their salt form, which is many times their Na+ or K+ salt form. Thus, use of a chelating agent, although reducing precipitation effects from certain metal ions, can actually exacerbate the precipitation effects of alkali metal ions. Sometimes the free acid or ammonium salt forms of chelating agents can be used to avoid this issue, at least in principle, but the free acid and/or ammonium salt forms of many chelating agents are either unknown or not commercially available at a reasonable cost. Furthermore, many common chelating agents are not biodegradable or present other toxicity concerns that can make their use in a subterranean formation problematic.